The present invention relates in general to the control of distortion during high temperature processing, and, more particularly, in the sintering process of metallized multilayer ceramic (MLC) substrates.
In the manufacture of MLC substrates, ceramic greensheets are formed from a casting slurry. The individual ceramic greensheets are personalized with via holes and conductive metal. The ceramic greensheets are then stacked together in a predetermined design sequence to form a green ceramic laminate. After the greensheets are stacked, heat and pressure are applied to the greensheets to provide a green ceramic laminate with continuous conductive metal wiring whose layers will remain contiguous during subsequent processing. This process of applying heat and pressure to the stacked greensheets is called lamination. The green ceramic laminate is then fired at high temperature in a process called sintering.
During the sintering process, employed primarily for densifying the ceramic and the conductive metal materials in MLC substrates, large volume shrinkage of the MLC substrate typically occurs. In general the ceramic and conductive metal materials have wide variations in physical and transport properties. The onset of densification and the densification profiles between the ceramic and metal differ widely as well. These variations result in distortion in the pattern of the conductive metal features as well as distortion in the substrate body shape. Distortion is a general term for the deviation in post sinter dimensions from ideal design dimensions. Distortion in the body shape includes deviations in surface flatness called camber. Distortion control requires the conductive metal and the ceramic material to have similar rates of shrinkage. However even with careful selection of materials, variations in material from lot to lot can result in unpredictable shrinkage. Shrinkage dissimilarities result in post sinter distortion. In MLC substrates this distortion can manifest itself as substrate warping, substrate camber and variations in the height of surface metal pads. This distortion results in rework and added cost since increasingly tight dimensional control is required for the assembly of electronic devices and packages.
The process of manufacturing MLC substrates involves multiple processes which directly impact the product dimensions during the sintering step. Extensive effort is expended at increased cost to control the MLC substrate post sinter dimensions. Advances in microelectronic technology have continuously increased the number of chip I/O (input/output) while decreasing the corresponding chip size. This creates a demand for MLC substrates with decreasing top surface metal (TSM) interconnect dimensions. In addition, cost reduction has driven reduced product sizes with a corresponding demand to increase MLC substrate bottom surface I/O pad density. Therefore, there is a need for cost-effective distortion control in MLC substrate manufacturing.
There are a number of methods employed currently to control substrate dimensions during MLC substrate manufacturing. An additional sinter process under pressure is applicable to all designs and will reduce ceramic distortion. However this process is an expensive cost adder which also results in additional product yield loss. Tailoring the type of conductive metal used throughout the substrate may be employed to control distortion after a design is complete, but this is only useful to control global distortion. This solution is not comprehensive and does not address the problem of random or local distortion. Green sheet stack lamination pressure adjustment is another technique to control distortion. However this technique is effective only in controlling global expansion. Finally, product redesign can be used to reduce distortion by adjusting the conductive metal distribution in key areas. However, this is undesirable since it is very costly and impacts new product time to market. It is not always possible to redesign as it is a trial and error technique and might compromise performance. The existing procedures and models used to control product dimensions are not fully predictive, and are therefore not dependable.
There are methods proposed by others to improve the dimensional control of electronic packages. Robbins et al. U.S. Pat. No. 5,801,073, the disclosure of which is incorporated by reference herein, discloses a method for producing an electronic packaging device made of dissimilar materials within a package. Robbins discloses a method to achieve minimal overall shrinkage of the package by the use of a high purity reaction bonded silicon nitride as a dielectric ceramic material.
Mori et al. U.S. Pat. No. 5,370,760, the disclosure of which is incorporated by reference herein, discloses a method to reduce the distortion of the metallized features in a ceramic laminate during the lamination process prior to sintering. Mori discloses the use of a die assembly, which is a tool, having an outer portion and an inner portion which can compress the outer peripheral portion of the laminate to a higher degree than the central portion of the laminate. This disclosure does not address the control of distortion induced during the sintering process.
Notwithstanding the prior art there remains a need to control the dimensions of MLC substrates already designed, but which fail to meet their post sinter dimensional requirements, and whose local or random distortion is not amenable to the existing dimensional control methods.
The inventors have discovered that the addition of properly tailored non-densifying structures, such as discontinuous thin metal structures, to the green ceramic laminate improves the local dimensional control of the ceramic product during the sintering process.
In one embodiment, the invention provides a method to control local post sinter dimensions of free sintered multilayer ceramic substrates by placing a mostly non-reacting discrete thin metal structure in the green ceramic laminate prior to sintering. One or several non-densifying structures are placed on one or more personalized ceramic greensheets which are then stacked and laminated to form a green ceramic laminate. The laminate is then sintered and the non-densifying structure will control the local dimensions of the free sintered multilayer ceramic substrate.
In another embodiment, the invention provides a method to control local post sinter dimensions in MLC substrates manufactured as a multi-up laminate, i.e., a laminate containing more than one individual product substrate, by placing at least one discrete non-densifying structure in the kerf area between the individual product ups prior to sintering and then separating the discrete non-densifying structure from the product using post sinter wet sizing. One or more non-densifying structures are placed on one or more multi-up personalized ceramic greensheets in the kerf area between the individual product samples. The multi-up personalized ceramic greensheets are stacked and laminated to form a multi-up green ceramic laminate which is then sintered wherein the non-densifying structures will control the local dimensions of the multilayer ceramic substrate. After sintering, the substrate (sintered laminate) will be sized into the individual product samples separating the non-densifying structures from each individual multilayer ceramic substrate.
In another embodiment of the invention a discrete non-densifying structure is used to control the local post sinter distortion of a multi-up multilayer ceramic substrate and another discrete non-densifying structure is used to control the local distortion within the individual product samples within the MLC substrate during sintering. A first discrete non-densifying structure is placed on one or more multi-up personalized ceramic greensheets in the product area of the individual ups. A second discrete non-densifying structure is placed on one or more multi-up personalized ceramic greensheets in the kerf area between the individual product ups. The multi-up personalized ceramic greensheets are stacked and laminated to form a multi-up green ceramic laminate which is sintered wherein the first and second discrete non-densifying structures will control the local post sinter distortion of the multi-up multilayer ceramic substrate. Post sintering, the multi-up substrate is sized to form individual product substrates and the second discrete non-densifying structure is separated from the individual ceramic substrate and the first non-densifying structure will remain in the ceramic substrate. The second discrete non-densifying structure may also be separated from the multi-up green ceramic laminate prior to sintering.